Led by Rapinoe, the tournament's top scorer with three goals, the United States finished with a maximum nine points with three wins from three matches in the four-nation round-robin tournament.
Second place went to Brazil, who capped their tournament with a 2-0 victory over Canada earlier on Wednesday at Exploria Stadium in Orlando.
Canada finished third with three points and Argentina, in their first appearance, came away with zero points.
Argentina, the only team in the event outside the top-10 in the FIFA rankings, were a last-minute replacement for Japan.
They had an early setback with three players ruled out of their opening match just hours before kickoff because of Covid-19 protocols.
They were overwhelmed early by a United States team that scored four goals in the first half.
The first two came from Rapinoe, who tapped in a ball from Rose Lavelle in the 16th minute.
The 2019 FIFA Player of the Year doubled the lead in the 26th when she finished off a through ball from Carli Lloyd.
"We wanted to come out fast as always," Rapinoe said. 'It's always dangerous to leave teams hanging around."
Lloyd herself made it 3-0 in the 35th off a cross from Kristie Mewis on the left side of the box.
Mewis made it 4-0 in the 41st.
Alex Morgan lofted a shot over Argentine goalkeeper Solana Pereyra in the 84th -- her first goal since the birth of her daughter, Charlie, in May.
Christen Press finished off the scoring with a header in the 88th.
The reigning World Cup champions haven't lost a game since a defeat to France in January 2019.
In the first match of the day's double-header, goals from Debinha and Julia Bianchi lifted Brazil over Canada.
Debinha scored her second goal of the tournament to put the Brazilians up 1-0 in the 15th minute, knocking in a rebound after a shot from Adriana ricocheted off the post.
Brazil, guided by former US coach Pia Sundhage, doubled the score in the 39th, taking advantage of a defensive lapse by the Canadians, who twice failed to clear a ball in their area.
It ended up at the feet of Bianchi, who fired a shot into the top corner of the net.
Standout matches today